ALLUVIAL FAN SEDIMENTS INTRODUCE LOW FREQUENCY NOISE INTO GRAVITY ANOMALIES

3D geologic maps and models often rely on gravity modeling to characterize key geologic structures within the study region, such as basin depths, fault offsets, and fault dip. Gravity modeling of such subsurface geologic structure generally assumes either homogeneous or spatially uncorrelated densities within modeled rock bodies and overlying sediments. This assumption allows modeling to focus on the shape of the subsurface bodies, for example, body depth or fault dip, which then underpin subsequent structural interpretations. Rock bodies and sediments, however, show both a range of densities and spatial correlation of these densities, in both surface and drill-hole samples. The spatially correlated densities add low-frequency noise to the models that is difficult to detect and characterize, and which can lead to misinterpretations of the subsurface structure.

Detailed drill-hole data and geologic mapping of alluvial sediments in the southwestern USA provide an opportunity to study the effects of spatially correlated densities on structural interpretations based on gravity. Gaussian random field modeling of the distribution of density within alluvial fan sediments indicates low frequency noise is likely present in measured gravity anomalies, contradicting the common attribution of the lower frequencies in gravity anomalies solely to deeper geologic structures. This low-frequency noise increases in power with an increase in alluvial fan sediment thickness. Its presence increases the ambiguity of interpretations of subsurface geologic body shape, such as basin analyses that attempt to quantify concealed fault depths, offsets, dip angles. In the southwestern United States, where basin analyses are important for natural resource applications, such ambiguity increases the uncertainty of geologic structure depicted in 3D geologic maps, and of subsequent process models that are based on such 3D geologic maps.